ATTRACTION". 1}} 



It is this tendency to recede from the centre 

 which is called the centrifugal force : the power 

 which draws it to the centre is called the centri- 

 petal force. 



If the direction of the forces continue the same, 

 and if one of them is accelerating or retarding, the 

 body will likewise describe a curve. Thus if a 

 ball be projected from a cannon, it receives an 

 impulse, which, if there was no resistance from the 

 air, and if it were not acted upon by gravity, 

 would cause it to move always in a straight line : 

 but as soon as it leaves the mouth of the cannon, 

 gravity acts upon it in a direction perpendicular to 

 the ground, and continuing to do so, becomes an 

 uniformly accelerating force : hence the ball comes 

 at length to the ground, having described in a 

 course a curved line. 



If the air were wanting, it can be calculated 

 that this curve would be a parabola ; and this is 

 the foundation of the theory of projectiles and the 

 art of gunnery. But the resistance of the air and 

 other causes occasion projected bodies to deviate 

 considerably from the parabolic curve, and render 

 it more difficult than it would otherwise be to cal- 

 culate the distance to which they may be sent. 



Although it has been stated above, that probably 

 all bodies have attraction, yet in several acknow- 

 ledged species of matter no weight is discoverable. 

 Matter has accordingly been divided also into pon- 

 derable and imponderable. 



The first consists of all known solid bodies 

 and non-elastic fluids, together with air or 

 gas, all of which have a sensible weight or 

 gravity. The second includes caloric or the 

 matter of heat, light, electricity, the magnetic 

 fluid, &c. ; none of which can be discovered to 



c 2 



